linux-ext4 - Re: Enable asynchronous commits by default patch revoked?

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4A92DDFA.1040906@redhat.com>
Date:	Mon, 24 Aug 2009 14:37:46 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Andreas Dilger <adilger@....com>
CC:	Theodore Tso <tytso@....edu>,
	Christian Fischer <Christian.Fischer@...terngraphics.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Enable asynchronous commits by default patch revoked?

Andreas Dilger wrote:
> On Aug 24, 2009  09:34 -0400, Theodore Ts'o wrote:
>   
>> On Mon, Aug 24, 2009 at 10:33:10AM +0200, Christian Fischer wrote:
>>     
>>> I try to figure out reasonable mount options for ext4.
>>>
>>> I've seen a "Enable asynchronous commits by default" patch from Sun, 21 Sep 
>>> 2008.
>>>
>>> Why is it revoked?
>>>       
>> It patch was never merged because the ayschronous commits feature
>> disabled all write barriers, so under heavy workloads a power failure
>> could cause data loss.
>>
>> No one has gotten around to looking at this closely; I think adding a
>> strategically placed blkdev_issue_flush() will allow us to safely
>> enable this feature, but it needs careful study.
>>     
>
> I don't think that was the issue, but rather that we wanted to have
> per-block checksums in order to handle the case were some block in
> transaction A is causing a transaction checksum failure, yet transaction
> B has already committed and begun checkpointing.
>
> One option discussed was to add a lightweight 16-bit checksum (e.g. TCP
> checksum) to the high bits of the t_flags of the block tag.  The checksum
> doesn't have to be very strong since the whole-transaction checksum will
> be the primary point of validation.
>
> Cheers, Andreas
> --
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.
>
>   

I still don't trust the logic. Seems like a very complex (and really 
non-intuitive - async and commit, really?) thing to support for a 
marginal performance impact. Any blkdev_issue_flush() call would dwarf 
the advantage of the async bit of the commit.

If we are looking to better support workloads that suffer from 
journalling, I suspect that we have more natural ways to do that...

ric

--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html